1 Introduction

Although ordinary Portland cement (OPC) is the most commonly used binder in concrete manufacturing, its highly energy-intensive production is responsible for the 5–8% of global anthropogenic CO2 (Ammenberg et al. 2015). OPC production is also thought to contribute for in between 74 and 81% of the total carbon footprint of concrete (Blankendaal et al. 2014; De Schepper et al. 2014; Flower and Sanjayan 2007; Turner and Collins 2013). The causes of the high CO2 emissions during OPC production are identified in (i) the calcination of limestone, which decomposes the CaCO3 contained in the limestone into CaO and CO2 (Worrell et al. 2001), and (ii) the high energy required to heat the raw materials at temperatures greater than 1400 °C (Huntzinger and Eatmon 2009).

Academic and industrial researches are currently committed to find alternatives to OPC, which can lower the environmental footprint of concrete (Ishak and Hashim 2015). Among these alternatives, some industrial residues, called supplementary cementitious materials (SCMs), can partially or completely substitute OPC in concrete production. Since SCMs are mostly residues coming from other industries, the use of SCMs can also enhance industrial symbiosis between the cement and the other industries (Ammenberg et al. 2015). One of the most commonly used SCMs is the ground granulated blast furnace slag (GGBFS), a by-product of pig iron produced during the manufacturing of steel in a blast furnace (Crossin 2015). Thanks to its amorphous structure, which gives a latent hydraulic reactivity, GGBFS can be blended with OPC up to a certain ratio, producing the so-called blended concrete. It has been proved that the use of GGBFS mixed with OPC can improve concrete technical properties like strength, permeability, and corrosion resistance (Shi and Qian 2000; Song and Saraswathy 2006; Yi et al. 2012).

Despite the successful implementation of symbiosis between steel and cement industries through GGBFS, the potential for the valorisation of other residues from steel production is not fully explored at present. Stainless steel slag (SSS), for instance, a residue produced during the stainless steel making process, has the potential to be used in alternative cement production. However, since chromium is an essential constituent of stainless steel, a fraction of it appears also in the SSS, together with other heavy metals, posing environmental and health threats (Huaiwei and Xin 2011). The chromium content has historically limited the valorisation of SSS. Consequently, new processes are needed to reduce or mobilised the leachable chromium and to make SSS recyclable in new construction materials (Adegoloye et al. 2015).

In particular, AOD-slag is a SSS produced in the argon oxygen decarburisation (AOD) furnace, where stainless steel is commonly refined. AOD-slag occurs in a very fine texture (a few μm diameter), giving to the slag the shape of a fine powder. The fine texture is due to a process called “dusting”, in which the dicalcium silicate (C2S) contained in the slag undergoes several polymorphic transformations that cause a volume expansion and a consequent pulverisation of the slag (Kim et al. 1992). The powder shape makes the handling of the slag difficult due to the risk of heavy metals leaching. Considering that 270 kg of AOD-slag is produced per 1 t of stainless steel, the massive quantities and the powder shape make AOD-slag management problematic from an industrial and environmental point of view (Zhao et al. 2013).

In order to avoid the problem of dusting, boron oxide (B2O3) is commonly added during the cooling process of AOD-slag in a quantity equal to 2% of the total mass of the slag (Durinck et al. 2008). Boron oxide stabilises the C2S, thus preventing the formation of the fine particles. Stabilised AOD-slag grains present a bigger texture (few mm) and a more stable chemical status, which allows their disposal in hazardous waste landfills or their reuse as low-quality aggregates, especially for road construction. The valorisation as low-quality aggregates represents, however, a low-value application with respect to the high-quality oxides (CaO, MgO, AlO2) contained in the AOD-slag, whose chemical potential can be activated and exploited (Salman et al. 2014a). According to Faraone et al. (2009), the CaO content of AOD-slag is closer to one of the GGBFS and OPC than to one of the natural aggregates (NA). However, even if AOD-slag and GGBFS present a similar chemical composition, the main difference lies on the phase composition. GGBFS are vitreous/highly amorphous, while AOD-slag presents a highly crystalline structure that is mostly considered non-hydraulic. Therefore, AOD-slag cannot be simply blended with OPC clinker, but further treatments are required in order to activate its binding properties.

Recent research investigates the potential of AOD-slag and other crystalline SSS to be used as binders (Baciocchi et al. 2010; Faraone et al. 2009; Iacobescu et al. 2016; Kriskova et al. 2012; Motz and Geiseler 2001; Panda et al. 2013; Salman 2014; Santos et al. 2013; Setién et al. 2009; Sheen et al. 2013). In particular, two different but equally promising routes are (i) the activation of AOD-slag as a binder through alkali activation and (ii) the creation of solid carbonated blocks through the carbonation of the slag.

An alkali-activated material is any binder system derived from the reaction between an alkali metal source (alkali hydroxides, silicates, carbonates, sulphates, aluminates or oxides) with a solid silicate powder, as for instance, an aluminosilicate-rich precursor such as a metallurgical slag, natural pozzolan, fly ash or bottom ash (Provis and van Deventer 2014). Carbonation refers to the reaction of CO2 with alkaline divalent cations from natural ores or alkaline solid waste, such as steel slag and fly ashes, to produce stable carbonate minerals (Pan et al. 2016).

The valorisation of AOD-slag through either alkali activation or carbonation raises many technical issues that have been described in the work of Salman et al. (2014; 2014a, 2014b, 2015, 2016). However, another prerequisite for the use of AOD-slag to substitute OPC binder is its environmental acceptability. As a follow-up of the above -cited work of Salman, the present paper uses attributional life cycle assessment (LCA) to assess the environmental impacts of newly developed construction blocks (called from now on SSS-blocks), produced through alkali activation and carbonation of AOD-slag. More in detail, three different SSS-blocks are analysed, one produced through alkali activation and two produced through carbonation. To better understand the trade-off between the environmental costs of AOD-slag valorisation and the environmental benefits of potential OPC substitution, the environmental performances of the SSS-block production are compared to the ones of traditional OPC-concrete.

Although in Salman et al. (2016), LCA was already used as a first attempt to analyse the potential environmental performances of alkali activation and carbonation using SSS as precursor; the scope of the work of Salman was a more wide analysis on the technical, environmental and economic challenges in the development of the SSS-block technology. Therefore, the LCA presented in the current paper wants to deepen the environmental results of Salman et al. (2016), to analyse the environmental performances of SSS-blocks at various levels (midpoint and endpoint) and to highlight the environmental hotspots in the production process. In order to avoid reproduction with the above-mentioned work of Salman, the present study only includes a concise description of the technical process that contains structural elements needed in the LCA analysis.

1.1 Literature review

Table 1 lists some of the most recent LCA studies analysing different alternative solutions to produce low carbon cement and concrete from industrial residues. A consistent number of studies focused on the partial or complete substitution of OPC with industrial SCMs (GGBFS or fly ashes), concluding that the OPC substitution with SCMs is a promising low-cost solution to radically decrease CO2 emissions, but its development has been limited by standardisation and availability of alternative materials. (Feiz et al. 2015; Habert et al. 2010a; Huntzinger and Eatmon 2009; Van den Heede and De Belie 2012). However, only a few studies are currently available on the environmental implications of the alkali activation and carbonation of metallurgic slags other than GGBFS.

Table 1 Literature review of previous LCA studies on alkali activation and carbonation
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Regarding previous LCA on alkali activation process, the few available LCA studies refer to alkali activation applied to different precursors (BFS, fly ash, metakaolin). It is worth to mention that a complete literature review on this field is hampered by the lack of a universally accepted terminology. Most of the available LCA studies refer to the alkali-activated materials as geopolymers. According to the State-of -the-Art Report RILEM TC 224-AAM (Provis and van Deventer 2014), geopolymers represent a subset of the broader category “alkali-activated materials”, where the precursor is almost exclusively aluminosilicate with a low amount of available calcium. However, this definition is not universally accepted since, as stated by Davidovits (2008), alkali-activated materials are not polymers, and therefore, geopolymers are not a subset of alkali-activated materials. Consequently, there exists a plethora of different names applied to very similar materials that may lead to confusion among the readers. In general, regardless their definition as geopolymer or alkali-activated materials, the previous LCA studies focused only on some of the environmental aspects, as global warming potential (Duxson et al. 2007; Habert et al. 2011), abiotic resource depletion and cumulative energy demand (Weil et al. 2009). All studies agreed that alkali-activated materials reduce the CO2 emissions within a range of 40 to 70% compared to OPC, while similar impacts are caused in abiotic resource depletion and cumulative energy demand. However, this result is only valid if no impact is allocated to the industrial residue acting as a precursor. If the impacts of the industrial process producing the residue are allocated by mass to the precursor, then the final results are completely reverse and geopolymers resulted to have higher impacts than OPC-concrete (Habert et al. 2011).

Regarding the carbonation process, many previous LCA studies focused on carbonation of minerals, but only a few studies are focused on carbonation of steel residues (Pan et al. 2016; Xiao et al. 2014). Results from these studies concluded that the higher is the CO2 capture capacity of the carbonation process, the higher is the energy required. Therefore, depending on the efficiency of the process, the amount of electricity required could offset the environmental benefits deriving from the CO2 uptake. However, the adverse impacts due to electricity consumption could be compensated by the utilisation of the carbonated residue as a supplementary construction material (Pan et al. 2016). Even when the environmental analysis is enlarged to other environmental categories (i.e. ecosystem quality and human health), the energy consumption remains the key factor affecting the environmental balance of the carbonation process (Xiao et al. 2014).

2 Materials and methods

2.1 AOD-slag valorisation in SSS-blocks

The slag used for the development of the SSS-blocks is obtained from a Belgian stainless steel plant, and it is sieved with a 500-μm sieve for carbonation and 160 μm for alkali activation. The average oxide composition of the two fractions of the slag is similar, and it is reported in Table 2.

Table 2 AOD-slag composition
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2.2 Life cycle assessment

2.2.1 Goal of the study, functional unit and system boundaries

The goal of the LCA is to assess the environmental impacts of SSS-blocks, made through activation of AOD-slag from stainless steel production. This product-level analysis will help to identify possible hot spots and consequently to improve the environmental performance of the SSS-block production processes.

More specifically, the LCA assesses the environmental impacts during the production of three different types of SSS-blocks: (i) an alkali-activated block (AA-block), produced by mixing the AOD-slag with alkali activators (sodium silicate and sodium hydroxide); (ii) a slow-carbonated block (SC-block), produced through the carbonation of AOD-slag with a pure CO2 stream in a carbonation chamber, kept in standard condition (22 °C, 1 atm) for 7 days; (iii) a fast-carbonated block (FC-block), produced through the carbonation of AOD-slag with a pure CO2 stream in a carbonation reactor, operating at 80 °C and 8.3 atm for 2.5 h in a 100-vol% CO2 environment.

The produced blocks were tested for their properties related to compressive strength (as per EN 196-1:1994), thermal conductivity (as per ISO 8302:1991), freeze-thaw resistance (as per NBN B 27-009) and heavy metal and metalloid leaching (as per EA NEN 7345) on at least three samples. A more detailed analysis of the results of these tests is available in Salman et al. (2014a, 2014b, 2015).

The three SSS-blocks present a compressive strength between 15 and 25 MPa/m2. This compressive strength is comparable to, or higher than, some of the commercially available OPC-concrete blocks. In particular, paver OPC-concrete is used to form a segmented paver surface, and its compressive strength falls in the range than one of the SSS-blocks (Fig. 1).

Fig. 1
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SSS-blocks vs paver OPC-concrete

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The functional unit represents the product ability to perform a given function, and it provides a reference to which all the inputs and outputs are referred. When using LCA to compare different products, a common functional unit must ensure comparability among the analysed alternatives. As described before, the compressive strength of the SSS-blocks is comparable to one of the paver OPC-concretes. Therefore, the presented LCA compares 1 m2 of SSS-blocks with 1 m2 of OPC-concrete, which is able to provide the same compressive strength. The compared surface is made of 50 blocks, each measuring 20 cm (length), 10 cm (width) and 5 cm (thickness).

Three different scenarios are analysed, corresponding to three different valorisation routes:

  • Scenario 1, alkali-activated-blocks (S-1, AA-blocks)-AOD-slag is valorised through alkali activation to produce AA-blocks.

  • Scenario 2, slow-carbonated-blocks (S-2, SC-blocks)-AOD-slag is valorised through carbonation in a carbonation chamber, to produce SC-blocks.

  • Scenario 3, fast-carbonated-blocks (S-3, FC-blocks)-AOD-slag is valorised through carbonation in a carbonation reactor, to produce FC-blocks.

The system boundaries of the considered scenarios for SSS-block production and paver OPC-concrete are illustrated in Fig. 2. As the AOD valorisation routes avoid the stabilisation through boron and the consequent low-quality recycling of the AOD-slag, the avoided use of boron and the avoided transport of stabilised AOD-slag to low-quality applications are given as a credit (negative value) to the AOD-valorisation processes. At the same time, NA must replace the AOD-slag in low-quality applications. Therefore, the process of production and transport of NA to low-quality applications must be included in the study. In reality, other industrial waste is available to replace AOD-slag in low value applications (e.g. construction and demolition waste). However, to keep the simplicity of the study, NA is considered as the only alternative to AOD-slag in low value applications.

Fig. 2
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System boundaries for LCA analysis

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The systems for both SSS-blocks and OPC-concrete consider a cradle-to-gate analysis, including only the production phase. The use phase and the end-of-life phase of SSS-blocks are excluded from the analysis, since empirical data on the possible long-life behaviour of SSS-blocks is still missing, due to the early stage of technology development. On top of that, a similar durability can be assumed between SSS-blocks and the OPC-concrete, and both materials can be considered as inert waste at their end-of-life (Salman 2014). To conclude, if the compared materials show similar functional properties and durability during the use and the end-of-life phases, then the limitation of the study to a cradle to gate analysis is valid (Habert et al. 2011).

2.2.2 Life cycle inventory

The life cycle inventory (LCI) phase estimates the consumption of resources, the quantities of waste flows and emissions caused during a product’s life cycle (Rebitzer et al. 2004). Therefore, the LCI phase creates a list of inputs and outputs related to the functional unit chosen, and it represents the basis for the calculation of the environmental impacts. The LCI for the presented study has been implemented on Gabi version 8.0.0.247, using Ecoinvent database v3.3 as the reference to model the background processes (materials, fuel and electricity sources). The electric mix used for all the process is referring to the Belgian electric mix 2017 (Elia 2017), which mainly consists of 46.6% nuclear, 26.5% gas, 11% renewables and 6.1% coal.

S-1: alkali-activated SSS-blocks

The process to produce AA-blocks considered in this paper is described in details in Salman et al. (2016, 2015, 2014b). In the initial step of the process, AOD-slags are mixed with commercially available 0–5 mm river sand in a weight ratio of 1/6 (slag to sand) and with sodium silicate and sodium hydroxide. After mixing, the AA-blocks are cured in a steam-curing chamber at 90 °C for 16 h. During the entire processing, the main electricity consumption is due to the energy needed for the mixer and the energy required to keep the stream curing chamber at high temperature.

Transport distances can vary greatly from case to case. However, some general assumptions can be made. These assumptions are common for both alkali activation and carbonation scenarios. When neighbouring industries start to exchange their secondary raw materials, transport distance reduction represents one of the main advantages. According to this prerequisite and considering a highly urbanised region, a distance of 10 km is assumed from the stainless steel plant producing the AOD-slag to the concrete factory, where SSS-blocks are produced. The avoided transport of stabilised AOD-slag to low-quality applications is assumed to be 50 km. Figure 3 summarises the input/output flows for the AA-block production.

Fig. 3
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System boundaries and processes of alkali-activated block production

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S-2 and S-3: carbonated SSS-blocks

Carbonation involves the reaction of carbon dioxide with alkaline materials, leading to the formation of stable carbonate products. Industrial carbonation simulates the natural weathering of silicates, in which natural occurring silicates fix atmospheric CO2 through the following chemical reaction, where the element X generally represents calcium or magnesium:

(1)

The carbonation process producing the SC and FC blocks is better detailed in Salman et al. (2014a, 2016). To produce the SC-blocks, the AOD-slag reacts with a pure CO2 stream in a carbonation chamber, kept in standard condition (22 °C, 1 atm) for 7 days. To produce the FC-blocks, the AOD-slag reacts with a pure CO2 stream in a carbonation reactor, operating at 80 °C and 8.3 atm in a 100-vol% CO2 environment. The higher values of temperature and pressure increase the kinetics of the carbonation process, allowing to complete the reaction in only 2.5 h. In Salman et al. (2014a), the uptaken CO2 is calculated to be the 15% of the mass of the slag. Within this 15%, the 2.25% of the total input of CO2 is not uptaken, and it is lost as direct process emission. Therefore, the total CO2 input of the process is calculated as the 17.25%, (15% uptaken plus the 2.25% lost in the process) of the mass of the slag.

The AOD-slag itself is used as aggregate; hence, the use of sand is avoided. For both SC and FC blocks, 18 wt% (of the slag) of water was added to the slag. Figure 4 summarises the input/output flows for the SC and the FC block production.

Fig. 4
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System boundaries and processes of carbonated block production

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Paver OPC-concrete

The concrete mixture (the share between binder, water and aggregates) of paver OPC-concrete is calculated based on the relation between the compressive strength and the cement content (Neville 2012; Ollivier et al. 2012) and on information collected directly from local concrete producers in Belgium. The transport distances for the NA and for the OPC to produce OPC-concrete are assumed equal to 50 km (Habert et al. 2010b; Martaud 2008; Mroueh et al. 2000). Figure 5 summarises the input/output flows for the paver OPC-concrete production.

Fig. 5
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System boundaries and processes of paver OPC-concrete production

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Finally, Table 3 summarises the inputs and outputs for all considered scenarios and the assumptions made for transport distances.

Table 3 Life cycle inventory table
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2.2.3 Allocation

The allocation issue for the use of industrial residues to produce new materials is an ongoing discussion in scientific literature. Early LCA studies on SCMs, especially focusing on GGBFS or fly ashes, did not attribute any environmental impact to the process generating these industrial residues (Van den Heede and De Belie, 2012). However, today, the partial substitution of OPC with GGBFS or fly ashes became a common practice and those materials are no longer considered as waste but instead as a by-product (Habert 2013). The ISO provides different possible solutions to deal with the allocation of industrial by-products. First, the allocation should be avoided by dividing or expanding the system. However, Chen et al. (2010) showed that system expansion is highly dependent on the point of view of the LCA practitioner, and it can present inconsistency when the main product and the by-product are considered within the same system boundaries. Second, when system division or expansion is not possible, other physical or economic parameters should be used to allocate the environmental burden. However, mass or economical parameters are not applicable in the case of metallurgic slags used in cement production. As demonstrated by Van den Heede and De Belie (2012), an allocation by mass poses an enormous environmental impact to the slags, which may discourage the concrete industry to use them as a cement replacement. An economic allocation allocates negligible impacts to the slags, due to the large differences in price between the main product and the slags. Chen et al. (2010) and Habert (2012) proposed alternative allocation methods for GGBFS valorisation in cement industry. These proposals are based on physical and economic empirical coefficients, as for instance, the equivalent binding capacity, which is available for GGBFS and fly ashes, but still unknown for AOD-slag.

Following the Waste Framework Directive 2008/98/EC and the recommendations put forth in the ISO 14041, an allocation coefficient should be indeed applied only if the waste can be considered as a by-product, while no allocation is advised if the waste is considered as an unintended residue.

A waste can be in fact regarded as a by-product if the following conditions are met:

  1. a)

    Further use of the substance or object is certain.

  2. b)

    The substance or object is produced as an integral part of a production process.

  3. c)

    The substance or object can be used directly without any further processing other than normal industrial practice.

  4. d)

    Further use is lawful, i.e. the substance or object fulfils all relevant product, environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

As reported in Iacobescu et al. (2016), SSS does not have the status of a by-product and it is today legally considered as waste material, since it does not meet all these conditions. Regarding the condition a, further use of AOD-slag is not certain because the research on AOD-slag valorisation is still in its early stage. Regarding condition d, the legislation regulating the use of AOD-slag is still missing and potential environmental and health consequences of AOD-slag valorisation are still under investigation. Therefore, for the LCA presented in this paper, the allocation procedure has been avoided and no impacts are attributed to the AOD-slag.

However, according to the level of desirability, a waste is considered as a by-product if it is sold with revenues or it is considered as an unintended residue if it is disposed with costs (Kronenberg et al. 2009). The desirability of waste, therefore, is not given only by its physicochemical nature, but it also depends on the economic circumstances, which are likely to change over time. Therefore, with future development of the AOD-slag valorisation technology, further research on allocation procedure may be needed. This will imply the need for empirical parameters applicable to AOD-slag. The determination of these empirical parameters for AOD-slag goes however beyond the scope of this paper.

2.2.4 Life cycle impact assessment

During the life cycle impact assessment (LCIA) phase, the potential impact from each inventory emission and material/resource flows is characterised and quantified, using specific characterisation models (Hauschild et al. 2013). Characterisation models can be grouped into two families: “problem-oriented” or midpoint, determining impact categories indicators at an intermediate position of the impact pathways, and “damage-oriented” or endpoint, aiming at more easily interpretable results in the form of damage indicators at the level of the ultimate societal concern (Jolliet et al. 2004).

The present study uses the CML 2016 method as characterisation model for the midpoint analysis CML is a LCIA calculation methodology developed by the Center of Environmental Science of Leiden University. CML proposes a set of impact categories and characterisation methods for the impact assessment step at a midpoint level. In order to confirm the findings of the CML midpoint analysis and to provide results that allow an easier comparison among scenarios, a further endpoint LCIA analysis is performed using Recipe endpoint as characterisation model.

In a LCA study, it is also important to check the magnitude of uncertainty. The uncertainty of results can be caused by inaccuracy or unrepresentativeness of data or modelling assumptions (Björklund 2002). Sensitivity analysis can reduce the LCA results uncertainty by evaluating the influence of input changes on the model’s results (Clavreul et al. 2012). Therefore, a sensitivity analysis is performed in Sect. 3.3 to assess the influence of some assumptions on the final LCA findings. In particular, the sensitivity analysis is performed by varying the transport distances in each scenario, while keeping all other parameters constant.

3 Results

3.1 Midpoint analysis

The estimated midpoint environmental impacts are reported in Table 4. As environmental impact categories are measured in different units, and to facilitate the comparison between the different scenarios, Fig. 6 shows the impacts relative to 100% for all categories.

Table 4 CML midpoint analysis
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Fig. 6
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Midpoint impact categories, as percentage of the largest impact

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Compared to paver OPC-concrete, the AA-blocks (S-1) present higher impacts in the categories ozone layer depletion (+ 25%), marine aquatic ecotoxicity (+ 4%) and freshwater aquatic ecotoxicity (+ 4%), while they show lower impacts in all the other categories. The SC-blocks (S-3) show higher impacts than the paver OPC-concrete in the categories terrestrial ecotoxicity (+ 17%), marine aquatic ecotoxicity (+ 17%), human toxicity (+ 65%) and fresh water ecotoxicity (+ 29%). The SC-blocks have also a negative impact in the categories global warming and acidification, meaning that the avoided impacts of CO2 uptake, boric oxide production and AOD-slag transport to low-quality recycling are higher than the caused impacts from the production of the SC-blocks. The AA-blocks and the SC-blocks do not have the highest contribution in any of the considered categories. On the other hand, the FC-blocks (S-4) caused the highest impact in the categories terrestrial ecotoxicity, ozone layer depletion, marine aquatic ecotoxicity, human toxicity, freshwater aquatic ecotoxicity, eutrophication and abiotic depletion fossil. Finally, paver OPC-concrete presents the highest impact compared to all SSS-blocks in the categories photochemical ozone depletion, global warming, acidification and abiotic depletion (elements).

3.1.1 Process contribution to midpoint results

To understand better the results presented in Table 4 and Fig. 6, it is also important to evaluate the contribution of the different processes to the final value for each category. Figures 7, 8, 9 and 10 show the relative contribution of different processes to the midpoint results for the AA-blocks, the SC-blocks, the FC-blocks and the paver OPC-concrete, respectively. The results for each impact categories are detailed in Table S1 in the Electronic supplementary material.

Fig. 7
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Alkali-activated blocks: relative contribution of the different processes to the midpoint results

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Fig. 8
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Slow-carbonated blocks: relative contribution of the different processes to the midpoint results

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Fig. 9
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Fast-carbonated blocks: relative contribution of the different processes to the midpoint results

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Fig. 10
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Paver OPC-concrete: relative contribution of the different process to the midpoint results

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Considering the results for AA-blocks in Fig. 7, the major contribution for all impact categories is given by the production of sodium hydroxide and sodium silicate, while minor contribution is given by the production and transport of the NA for low-quality applications. The left side of the graph in Fig. 7 represents the negative values, therefore the avoided impact. A major contribution to the avoided impact is given by the avoided production of boric oxide, despite the low quantity used (2 wt% of the AOD-slag). However, for all impact categories, the sum of the positive impacts is always higher than the sum of the avoided impacts.

For the SC-blocks (Fig. 8), the major contribution to the different impact categories comes from the production of pure CO2, with some minor contributions given by the NA production and transport. Among the avoided impacts, the avoided boric oxides production plays the major role in all categories, except for global warming potential, where the major avoided impact is given by the CO2 uptake (− 12 kg CO2-equiv.).

Considering the process contribution in the FC-block production (Fig. 9), the electricity consumption and the CO2 production give an important contribution to the final environmental impact, especially in the categories ozone layer depletion, global warming and abiotic depletion. Finally, as already proved by previous studies (see, for instance, Turner and Collins 2013), for each of the analysed categories, the OPC production contributes the most to the final environmental impact of paver OPC-concrete (Fig. 10).

The process contribution analysis for AA-blocks, SC-blocks and FC-blocks show consistency with previous LCA analyses on alkali activation and carbonation. Previous studies conducted on alkali activation of GGBFS and fly ashes showed, in fact, a similar performance in the midpoint category global warming, finding that concrete made with alkali-activated SCMs outperforms the OPC-concrete (Weil et al. 2009). In addition, Habert et al. (2011) and Duxson et al. (2007) identified also in the production of sodium silicate the main driving force in determining the environmental impacts of the alkali activation process, causing higher impacts than traditional OPC-concrete in midpoint categories other than global warming potential.

Regarding the carbonation, Kirchofer et al. (2012) proved that the carbonation of cement kiln dust, GGBFS and fly ashes shows a negative CO2 balance for a reaction at 25 °C (meaning more CO2 uptaken than emitted), while it becomes positive when increasing the temperature of the reactor. These results strengthen the importance of the trade-off between the additional reactivity gained with higher temperatures and pressures and the increase of energy consumption to reach these conditions.

3.2 Endpoint analysis

Table 5 reports the result of the Recipe endpoint analysis, while Fig. 11 shows the same results relating the highest impact for each endpoint category to 100%.

Table 5 Recipe endpoint analysis
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Fig. 11
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Recipe endpoint results

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In the endpoint categories, human health and resources, the FC-blocks have the highest contribution (4.42E−04 DALY and $1.087), followed by paver OPC-concrete (10% in human health and 32% in resources), SC-blocks (5% in human health and 23% in resources) and AA-blocks (3% in human health and 10% in resources). For the endpoint category ecosystems, the paver OPC-concrete is the material with the highest contribution, followed by the FC-blocks (84%) and the AA-blocks (18%). The SC-blocks are the only material presenting a negative value in the endpoint analysis in the category ecosystems.

3.3 Sensitivity analysis

The sensitivity analysis tackles a possible source of uncertainty, represented by the assumption made on transport distances during the LCI phase.

One of the main advantages of circular economy is that material exchange between two industries has normally a local or regional dimension, and it enables a reduction of transport distances, compared to primary raw materials (Ghisellini et al. 2016). Nevertheless, transport distances can highly vary from one case to the other, and material exchange between two industries might involve long transport distances. In order to assess the sensitivity of the LCA results to transport variation, a scenario (sensitivity scenario) with less favourable transport conditions for the AOD-slag valorisation is studied. In this scenario, the transport distance for the AOD-slag from the steel to the concrete factory producing the SSS-blocks is set at 100 km, while all the other distances are kept equal to the base case. Table 6 shows the variation of the Recipe endpoint LCA results for the transport scenario.

Table 6 Transport sensitivity analysis
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The endpoint category, resources, is the most affected by the increased transport distances, showing increments between 38% (S-2) and 9% (S-3), while ecosystems show increments between 20% (S-1) and 11% (S-3). Human health is the endpoint category less affected by the distance increase, with increments between the 21% (S-2) and only 1% (S-3). SC-block is the material that presents the highest increments in two of the three endpoint categories (human health and resources), while the FC-block is the material that is less affected by the increase of transport distances. Therefore, when a high amount of electricity is consumed during the production process, the influence of transport variation becomes less relevant.

Figure 12 compares the Recipe endpoint results of the new transport scenario with the Recipe endpoint results of the paver OPC-concrete. It becomes evident that an increase of transport distances does not change the final ranking of the analysed materials. The only significant increment is reported in the damage Resources for the SC-blocks, where the increment of 38% compared to the base case brings the final value up to 0.34 species year, which is equal to the value for paver OPC-concrete.

Fig. 12
figure 12

Sensitivity analysis on transport variation. Comparison between sensitivity scenarios and base case paver OPC-concrete. Results for recipe endpoint

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4 Result interpretation and limitations

The LCA results presented in this study show the potential of AOD-valorisation to reduce the environmental impacts, compared to the traditional paver OPC-concrete.

In particular, the valorisation through slow carbonation seems the most promising option in terms of impact reduction. The major negative environmental effect, represented by the production of the pure CO2, is offset by the environmental benefits due to the avoided boric oxide production, the CO2 uptake and the low electricity consumption. On the other hand, however, the slow carbonation requires a long process time (7 days) that might be not cost-efficient for an industrial implementation of the process. Raising the temperature and the pressure can sensibly increase the kinetics of the carbonation, reducing the process time to only 2.5 h. However, this hoists also environmental issues. The CML midpoint and Recipe endpoint analyses show that the electricity required to accelerate the process increases significantly the final environmental impacts. In some midpoint and endpoint categories, the final environmental impact of fast carbonation exceeds one of the paver OPC-concretes, offsetting the environmental benefits of slag recovery and of the avoided boric oxide production. In view of a future industrial development of the carbonation process, it is therefore fundamental to optimise the trade-off between acceptable process time and sustainable electricity consumption. Lowering the impact coming from the production of CO2 represents another valuable alternative solution to improve the environmental performances of the carbonation process. For instance, some studies have investigated the performances of the carbonation process by using CO2 coming from flue gas directly from the steel industry (Tian et al. 2013). However, the results are not satisfying because of the presence of impurities that inhibit the performance of the processes.

The valorisation through alkali activation shows a promising environmental performance, thanks to the impact reduction in many midpoint and endpoint categories, compared to traditional paver OPC-concrete. While the traditional paver OPC-concrete production is energy intensive and produces high quantities of direct emissions, the alkali activation shows low-energy requirements and no direct emissions. These factors, in combination with the avoided boric oxide production, make the alkali activation process environmentally attractive, especially in terms of CO2 reduction. However, it should be noticed that the alkali activation requires a certain amount of chemicals (alkali activators). The results of the LCA show that the production of these chemicals can negatively affect the environmental profile of alkali-activated materials, especially for some midpoint categories, where the impact of alkali activation is higher than the impact of paver OPC-concrete. Therefore, for a future industrial development, the quantity of alkali activators added to the process should be optimised and limited, in order to reduce its environmental impact. As an alternative, alkali activators could be recovered from glassy waste streams (Barbieri et al. 2000). However, at present, there is not an established procedure allowing the production of pure alkali compounds from waste streams.

The sensitivity analysis performed on transport distances shows how an increase of distances can have a negative effect on the final environmental performances of the SSS-blocks, but this negative effect does not influence the comparison with the environmental performances of the paver OPC-concrete.

It is also worth to highlight the limitations of the presented LCA results. First, the data used for the LCI are case-specific primary data collected from laboratory tests. However, the LCA results obtained at a laboratory scale might not be necessarily the same as the LCA results obtained at an industrial scale, since optimisation and other learning effects may occur (Shibasaki et al. 2006). These improvements may lead to a more efficient use of material or an increased energy efficiency of individual process steps. On the other hand, a variation in the composition of the slag batches may affect the industrial production of SSS-blocks with a stable quality (Salman et al. 2016). As the economic value of the SSS-blocks is low compared to the value of the stainless steel, the metallurgic process will indeed focus on the properties of the stainless steel, rather than on the quality of the SSS.

Another limitation of the LCA results on SSS-blocks is represented by some old Ecoinvent datasets used during LCI phase. For instance, the dataset for boric oxide production refers to data collected during the period 2000–2006, while the datasets for CO2, sodium hydroxide and sodium silicate production are valid until the year 2011. Even if these datasets are available in the most updated version of Ecoinvent (version 3.3, 2016), they might not be representative of today’s technologies.

Finally, an additional limitation of the LCA results is represented by the intrinsic limitation of attributional LCA when it comes to industrial decision making and evaluation of industrial symbiosis applications. As clarified by Marvuglia et al. (2013) and Vázquez-Rowe et al. (2013), an attributional approach provides a good environmental reporting and understanding of the main environmental impacts within the analysed production system. On the other hand, it omits the analysis of potential indirect effect engendered in the markets by the underlying actions. Therefore, attributional LCA results provide a good environmental analysis at a product level, which enables a reliable comparison between alternative products. However, from attributional LCA results, no conclusions can be made on the environmental consequences of product substitution.

5 Conclusions and future work

The production of ordinary Portland cement (OPC), a widely used binder in concrete production, is thought to be responsible for 5–8% of the global CO2 emissions. Therefore, the cement and concrete producers, together with the scientific community, are currently engaged in finding sustainable alternatives to OPC. Following the principle of industrial symbiosis, the present research has analysed the environmental performances of new construction materials using metallurgical slag as replacement of OPC. In particular, the AOD-slag, a residue from the stainless steel production, has been used to produce three different construction blocks, called SSS-blocks. The environmental performance of the SSS-blocks has been compared with the environmental performance of equivalent OPC-based construction blocks, used as paver concrete. The three SSS-blocks were produced separately by activating the AOD-slag through three different processes: alkali activation, slow carbonation and fast carbonation. From the results of the LCA, some conclusion can be drawn:

  • The valorisation of AOD-slag has the potential to lower some environmental impacts compared to paver OPC-concrete. For instance, the impact reduction can be significant in midpoint categories as global warming potential, where the analysis shows a reduction of 84% for alkali-activated blocks, of 35% for fast carbonated blocks and of 117% for slow carbonated blocks. At the same time, in some other midpoint and endpoint categories, the environmental impact of the SSS-blocks is higher than one of the paver OPC-concretes. For instance, the alkali-activated blocks show an increase of 58% in the midpoint impact category ozone layer depletion, the slow-carbonated block shows an increase of 45% in the midpoint category human toxicity and the fast-carbonated block shows increases of 90 and 68% in the endpoint categories human health and resources.

  • The production of alkali activators represents the environmental hotspot of the alkali activation process. Some effective solutions to lower the environmental impacts of alkali-activated materials rely on a more efficient use of alkali activators during the process or to a more sustainable alkali activator production process (as for instance, the possibility of recovery, the alkali activators from specific waste streams).

  • The slow-carbonated blocks represent the materials with the lowest environmental impacts, both at midpoint and endpoint analyses. However, the long process time required to complete the process makes the slow-carbonated blocks less attractive for possible industrial development. On the other hand, the possibility of increasing the kinetics of the process raises many environmental concerns, due to the electricity consumed to keep the high temperatures and pressures required. Therefore, it is fundamental to find the right balance between acceptable process time and electricity consumption. Another environmental hotspot for the carbonated blocks is represented by the use of pure CO2 streams. The recovery of CO2 from flue gas streams from other industries can represent an opportunity to reduce the overall environmental impacts of the process.

  • The sensitivity analysis on transport distances shows how increments up to ten times higher than the one assumed in the base case do influence the environmental profile of SSS-blocks. However, they do not affect the final comparison with paver OPC-concrete, showing that the benefits of AOD-slag valorisation are still valid even if high transport distances must be covered.

The LCA analysis highlights also important limitations on the data used as input and the database for chemicals and the LCA modelling:

  • The input data used in the LCA model are based on a case-specific development in laboratory tests. The possible upscaling of the processes could not only have positive effects, as an increased efficiency in the use of alkali activators and a more efficient uptake of CO2, but also it may present some risks due to possible variation of slag batch composition.

  • The database for chemicals used in the LCI phase is outdated, and it may not represent the today’s technologies.

  • Attributional LCA presents limitations when assessing the effect of product substitution into an economic system. Therefore, from the current analysis, no conclusions can be made on the environmental consequences for the steel and construction sectors when paver OPC-concrete is substituted with SSS-blocks.

A sustainable environmental profile and a promising economic plan are two fundamental prerequisites for industrial and market development of new products. Economically, to upgrade research and lab-based innovation into industrial products, an attractive business case is key. The economic potential of SSS-blocks is promising since low-value or even costly (disposed) residues are transformed into valuable new products. Therefore, an in-depth analysis of the costs and revenues from bringing SSS-blocks to the market would further the understanding of the potential and would narrow the remaining distance to the market for these sustainable construction materials. Environmentally, as proved in the presented LCA-study, the SSS-blocks represent a good opportunity to lower the impact of cement industry. Consequently, industrial and scientific research should focus on increasing the efficiency of the processes. In addition, to make the environmental analysis more accurate, there is an urgent need for updated environmental life cycle inventory dataset for alkali activators, boric oxide and CO2 production. Finally, the attributional approach should be integrated with a more systemic approach (e.g. consequential-LCA) able to widen the scope of LCA by assessing the possible environmental and economic consequences at a holistic system level.